Metastatic Urachal Carcinoma With Microsatellite Stability and Low Tumor Mutational Burden Achieving a Durable Response to Pembrolizumab After Platinum Failure

ABSTRACT Introduction Urachal carcinoma is a rare and aggressive malignancy with no established systemic therapy after platinum failure. Evidence for immune checkpoint inhibitors (ICIs) remains limited, particularly in tumors without established favorable biomarkers such as microsatellite instability‐high status or high tumor mutational burden. Case Presentation A 55‐year‐old woman with metastatic urachal adenocarcinoma progressed after seven cycles of gemcitabine–cisplatin. Pembrolizumab (200 mg every three weeks) was initiated, resulting in a durable partial response lasting over 12 months without immune‐related adverse events. Molecular profiling showed microsatellite stability, low tumor mutational burden, and insufficient tissue for (Programmed death‐ligand 1) PD‐L1 assessment. Conclusion This case demonstrates durable disease control with pembrolizumab in metastatic urachal carcinoma despite microsatellite stability and low tumor mutational burden. These findings suggest the potential efficacy of pembrolizumab in selected patients with platinum‐refractory disease.